Characterization of some major coagulation factors of lamprey blood, one of the most primitive vertebrates, was performed. This included elucidation of the molecular properties of lamprey fibrinogen, as well as its individual subunits, (chains), via sedimentation-equilibrium, column chromatography and gel electrophoresis. The kinetic-molecular properties give a molecular weight of 350,000 plus or minus 12,000 and indicate an unusual structure. Thrombin-like enzymes were reacted with this protein and the resulting fibrins examined by electron microscopy.